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The CENIM-LNETI process: a new process for the hydrometallurgical treatment of complex sulphides in ammonium chloride solutions
tlydrometallurgy, 28 ( 1992 ) 149-161
149
Elsevier Science Publishers B.V., A m s t e r d a m
The C E N I M - L N E T I process: a new process tbr the hydrometallurgical treatment of complex sulphides in ammonium chloride so, utions J.L. Limpo ~, J.M. Figueiredo ~, S. Amer a and A. Luis ~ ~Centro Nacional de Investigaciones Metalurgicas (Ct_'NIM), C.S.L C., A venida de Gregorio del Amo 8, 28040 Madrid, Spain bLaboratorio Nacionai de Engenharia e Tecnoh~:~:iaIndustrial (LNETI), Lisbon, Portugal (Received January 24, 199 !; revised version accepted July 22, 1991 )
ABSTRACT Limpo, J.L., Figueiredo, J.M., Amer, S. and Luis, A., 1992. The CENIM-LNETI process: a new process for the hydrometallurgical treatment of complex sulphides in ammonium chloride solutions. lirdrometallurgy, 28:149-161. A new process for the hydrometallurgical treatment of complex sulphides is presented. This process is based on an oxidizing leach of the sulphides in an ammonium chloride medium, which produces ammonia which forms metal ammine complexes with ith.~ solubilized metals (Cu and Zn ). The leaching occurs at a practically constant and neutral pH, arn~ produces a solution which is totally free of iron. Impurity elements, such as As, Sb, Bi and Sn, aTre detected in the solution only at trace levels. The presence of ammonia in the fertile leaching solutions facilitates its treatment by solvent extraction with acid cationic extractants, a stage in which tl~e leaching solution is regenerated. The paper presents a basic, general description of the process, ill us !rated with the results obtained using one of the bulk concentrates analyzed.
INTRODUCTION
Spain and Portugal have large deposits of complex sulphides, where the sulphides of non-ferrous metals are thialy dispersed through a pyrite matrix with a high level of association. These structural characteristics make it difficult to take full advantage of differen'tial flotation, as it is practically impossible to achieve high recoveries and hitChgrades of the individual concentrates at the same time. One solution to this problem is the recovery of the individual metals through treatment of the bulk concentrate. The main difficulty in this approach lies in the complex prc,blem of the treatment of these polysulphide bulk concentrates. Although many processes have been reported in the past, the treatment may still be consi~lered to remain unsatisfactorily resolved. In recent years, CENIM in Spain I~ad LNETI in Portugal have been work0 3 0 4 - 3 8 6 X / 9 2 / $ 0 5 . 0 0 © 1992 Elsevier Scit :ice Publishers B.V. All rights reserved.
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J.L. LIMPO ET AL.
ing on this problem in a joint project sponsored by the EC. The result of this bilateral collaboration has been the development of a new treatment procedure, the CENIM-LNETI process, which proposes an answer to the problem that seems to have clear advantages over existing processes. The basis of the process is an oxidizing leach in a chloride medium. Although there is an extensive body of literature on chloride leaching, there is practically nothing published on the use of concentrated solutions of ammonium chloride as the leaching medium [ 1 ]. The use of ammonium chloride confers special advantages on the process, making it particularly appropriate for the treatment of complex sulphide concentrates. The present article provides a general description of the process and discusses some of the most important results of the treatment of different bulk concentrates. HYDROMETALLURGICAL TREATMENT IN AMMONIUM CHLORIDE SOLUTIONS
The high solubilizing power of the chloride medium has attracted a great deal of attention in the study of the hydrometallurgical treatment of sulphide ores, particularly for the treatment of complex sulphides. As a result, several processes have been reported in recent years which basically differ in the use ofdifferent oxidating agents: Cu 2+ [2,3 ]; Fe 3+ [4-6]; O2 [7,8 ]; C12 [8 ]; etc. However, all use a leaching medium with a high concentration of chlorides, which is normally achieved through the addition of sodium chloride or, less often, by the addition of calcium chloride or a sodium chloride-.calciurn chloride mixture. In all these processes, the saline additive p,'oviJes a sufficiently high concentration of chloride ion to guarantee the solubl!ity of lead and silver. l h e addition of calcium chloride permits the control of sulphates in the solution. The CENIM-LNETI process uses a concentrated solution of ammonium chloride as the leaching medium and oxygen as the oxidizing agent. As well as providing an adequate concentration of chloride ion, the addition of ammonium chloride has other functions which are equally important and distinguish this process from the others. These characteristics are derived from the weak acid nature of the ammonium ion according to the reaction: NH~ ~ H + +NH3
(1)
The use of ammonium chloride thus has two advantages. Firstly, like the other chlorides, it provides an adequate concentration of chloride ion and, secondly, the ammonium ion is able to generate the protons required for the leaching of the sulphides, as well as producing ammonia. The resultant ammonia forms highly stable complexes with the majority of common metals (Cu 2+, Cu +, Zn 2+, Ag+, Hg 2+, etc. ), and this increases the solubilizing power of this leaching medium.
TREATMENT OF COMPLEX SULPHIDES IN AMMONIUM CHLORIDE SOLUI IONS
15 l
In the oxygen leaching of non-ferrous sulphides at moderate temperatures ( < !50°C) in an acid medium, the sulphur is essentially oxidized only to its elemental state. On the other hand, when the leaching occurs in a basic medium, the oxidation continues up to the formation of sulphates. When leaching takes place in ammonium chloride solutions, the leaching medium has a pH value which is very close to neutral. Under these conditions both possibilities play an important role. The reactions and mechanisms by which there is an attack on the different minerals are complex, and are analyzed in detail elsewhere [ 9 ]. Dispensing with the intermediate steps, leaching in an ammonium chloride medium develops according to a competition between the following two global reactions: MeS+2NH~- + ½02--,S+Me 2+ +HEO+2NH3
(2)
MeS + 202 ~ SO42- + Me 2+
(3)
Thus, in addition to solubilizing the non-ferrous metals, the leaching reactions also produce ammonia and sulphates which enter the solution. One of the most significant characteristics of leaching in an ammonium chloride solution is the practically constant and almost neutral pH, generally in the 6-7 range. In spite oi" the continuous generation of ammonia in the solution, the pH values does not vary significantly because copper and zinc form highly stable ammonia complexes. As a result, the ammonia formed during leaching mostly becomes combined in the form of metal ammines, and only a small quantity of uncombined ammonia in equilibrium remains as free ammonia with a high concentration of ammonium salts. This constitutes a buffer system which keeps the pH practically constant. The most important consequence of leaching in a neutral medium is the high purity of the solution obtained. Following leaching, the pregnant solution with the metallic values to be recovered has extremely low amounts of impurities and, in particular, is totally free of iron. Due to the high pH value of the solution, all the iron contained in the attacked mineral species (sphalerite, chalcopyrite and pyrite) remains in the residue as hydrated ferric oxide. Other minor elements, such as As, Sb and Bi, also stay almost totally in the leach residue, and are detectable only in trace amounts in the solution. Finally, another important characteristic of this leaching process is the need for a small initial concentration of dissolved copper. Without this metal, the leaching rate falls considerably, and practically stops at temperatures below 105 ° C. This behaviour is interpreted as indicating that the attack of the sulphides is via Cu 2+ ions, while oxygen re-oxidizes the Cu + ion which is formed. CONCENTRATES
The development ofthe CENIM-LNETI process was carried out using three
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J.L. LIMPO ET AL.
TABLE 1 Composition of the concentrates Element
Zn Cu Pb Fe S As Sb Bi Ag
Analysis (%) Sotiel
Aznalcollar
Ajustrel
33.5 3.4 7.8 17.1 34.6 0.28 0.32 0.01 0.018
29.6 3.8 9.9 16.5 33.8 0.28 0.40 0.12 0.023
19.5 8.3 6.1 21.3 36.3 0.30 0.13 0.12 0.012
different bulk concentrates: the Spanish concentrates of Sotiel and Aznalcollar and the the Ajustrel concentrate from Portugal. Table 1 sets out the composition of the major elements in the three concentrates. X-ray diffraction analyses indicated that the concentrates have similar mineral compositions, primarily: sphalerite, chalcopyrite, galena and pyrite. Table 1 shows that the most significant difference is the greater copper content, and thus lower lead and zinc contents, in the Portuguese concentrate. EXPERIMENTAL RESULTS
Leaching in ammonium chloride solutions The leaching of complex sulphide concentrates with oxygen in concentrated solutions of ammonium chloride produces the easy solublization of the important metal values, provided that there is copper in the leaching solution. The leaching rate is high in the presence of copper, even under operating conditions of moderate oxygen partial pressures and temperatures. As an example, the results from this type of leaching are set out in Fig. 1, where the percentage of leached metal is shown as a function of time for Zn, Cu, Pb and Ag. The raw material used in this test was the Sotiel bulk concentrate, using a pulp density of 100 g concentrate/kg water. The rest of the experimental conditions were the following: 105°C, 100 kPa oxygen partial pressure and an initial solution composition of 321 g ammonium chloride/ kg water and 1.5 g copper/kg water. As seen in Fig. 1, the solubilization rate for Pb and Ag is very fast and is considerably more rapid than that of Zn and Cu. A possible cause of the higher rate is the smaller granulometry of the galena compared to that of the sphalerite and chalcopyrite in these concentrates, although this fact alone is not
TREATMENT OF COMPLEX SULPHIDES IN AMMONIUM CHLORIDE SOLUTIONS
153
100
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9'o TIME (MIN)
Fig. !. Leached metals as a function of time for the Sotiel concentrate.
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60
90
120
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Fig. 2. Ammonia and sulphate produced versus time for the experiment outlined in Fig. 1.
sufficient to explain such a large difference. The maximum solubilization of Pb is reached in the first few minutes, after which there is a gradual fall in the percentage of solubilized metal as a function of time. This phenomenon has been repeated systematically in all the tests, and is a consequence of ce~ain hydrolysis reactions. Figure 2 shows the evolution with time of the ammonia and sulphate concentrations during the same test. The figure indicates that there is a continuous increasing trend in the sulphates, while the ammonia tends to reach a maximum value asymptotically. In order to clarify this effect, a series of tests was done with longer leaching times. It was found that a maximum value of ammonia in solution was reached and, thereafter, there was a fall in the amount of ammonia that intensified with increasing leaching time. This fall occurred as a result of the oxidation of the pyrite according to the reaction:
154
J.L. L I M P O ET AL.
FeS2 + 1 5 / 4 0 2
+
5/2H, O--,2H2 SO4
4- 1 / 2 F e 2 0 3 " H 2 0
(4)
The oxidation of pyrite occurs at a substantially lower rate than that of the non-ferrous sulphides. The reduction of the ammonia content thus begins only when a certain degree of exhaustion of these sulphides is reached. This occurs when the rate of ammonia production according to eqn. (2) is lower than that of acid production according to eqn. (4). Figures 3 and 4 represent the variation with time of the percentage of Zn and Cu leached at different temperatures in the 85-125 °C range. The remaining operating conditions were maintained constant at the following levels: oxygen partial pressure 100 kPa; pulp density 100 g concentrate/kg water; 1001. . . . . . . . . . . . . . . . .
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Fig. 3. Effect ot" temperature on zinc leaching kinetics. L,each conditions: !' o,= !00 kPa, [ NH,,CI ],=6.0 mol/kg !-i~O, [Cu ],= !.5 g/kg H~O and 100 g Sotiel bulk concentrate/kg H,O.
l O 0 ...............................................................................................................
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120
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Fig. 4. Effect of temperature on copper leaching kinetics. Leach conditions: Po,= 100 kPa, [ NH4CI ]0=6.0 mol/kg H20, [Cu ]o= !.5 g/kg H20 and 100 g Sotiel bulk concentrate/kg H20.
TREATMENTOF COMPLEXSULPHIDESIN AMMONIUMCHLORIDESOLUTIONS
155
ammonium chloride concentration 321 g/kg water; initial concentration of copper 1.5 g/kg water; and Sotiel concentrate. The leaching rate clearly in= creases with temperature, which is the most influential of all the variables. The results obtained for the other two concentrates show a similar behaviour except that higher leaching rates are obtained with the Ajustrel concentrate. This is possibly related to its higher chalcopyrite (i.e., copper) content. The percentage of solubilized Zn and Cu as a function of time is shown in Figs. 5 and 6, respectively, in a series of leaching tests at oxygen partial pressures ranging between 100 and 500 kPa. The temperature was maintained constant at 105 °C in all the tests. The influence of 02 partial pressure is obviously small. There is a slight increase in leaching rate when the pressure 100-80 r" U
,._, 60 I u
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120
Fig. 5. Effect of oxygen pressure on zinc leaching kinetics. Leach conditions: T= 105°C, [ NH.ICI ]o=6.0 mol/kg HaO, [Cu ]o= 1.5 g/kg H20 and 100 g Sotiel bulk concentrate/kg 1-120. 100 .............................................................................................
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Fig. 6. Effect of oxygen pressure on copper leaching kinetics. Leach conditions: T = I05°C, [NH4CI ]o = 6.0 m o l / k g H20, [Cu ]o = 1.5 g l k g 1-120) and I00 g Sotiel bulk concentrate/kg 1420.
156
J.L. LIMPO ET AL.
increases from 100 to 200 kPa, but beyond 200 kPa, oxygen partial pressure has no effect. The initial concentration of copper in solution has a negligible effect on the leaching rate provided a minimum of 1 g Cu/kg water is maintained. During leaching, a minimum degree of agitation is required to ensure the transfer of oxygen to the solution to oxidize the cuprous ion in accordance with its formation rate. An increase in stirring above this level has no influence on the leaching rate. However, a reduction in stirring below the critical level may have serious consequences because, when the oxygenation is incomplete, the copper is precipitated according to the following reaction: 2Cu + + S-,Cu 2+ +CuS
(5)
which stops the leaching reaction when all the copper is precipitated.
Acid and two stage leaching According to the results exposed above, the amount of ammonia produced during leaching reaches a maximum value, and then decreases with a prolongation of leaching time. The maximum value is reached for an average solubilization of the metal values ( Z n + C u + Pb) between 80% and 85% Beyond this point, when solubilization yield increases, the amount of ammonia begins to decrease; initially slowly, but beyond 90% yield faster. In the previous section lead solubilization was also seen to fall as the leaching time increased, because of certain hydrolysis reactions. These results indicate the advisability of a two-stage countercurrent leaching process involving an initial stage of neutral leaching in which the bulk concentrate is treated, and a second stage of acid leaching in which the residue from the first stage is exhausted. Table 2 sets out the results obtained using the Sotiel bulk concentrate in a series of two-stage leaching tests. The experiments were programmed in the basis of the results of the individual analyses of the neutral and acid leach. In all tests a oxygen partial pressure of 100 kPa was used. The temperature and time of each stage is indicated. In these tests, neutral leaching involved the treatment of the concentrate with the solution obtained from the previous acid leach, while the residue obtained in the first stage was treated to exhaustion in the acid leaching stage. In all cases, the acidity used was the stoichiometric equivalent of tile non-ferrous metals contained in the residues to be leached. The solubilization yields shown in Table 2 are calculated on the basis of the analytical values of the leach residues.
Treatment of the leaching solution One of the most interesting characteristics of the ammonium chloride leaching medium is the in situ production of ammonia. The ammonia combines with the dissolved metals according to the corresponding complexation
TREATMENT OF COMPLEX SULPHIDES IN AMMONIUM CHLORIDE SOLUTIONS
157
TABLE 2 Results of two-stage leaching tests Bulk concentrate: Sotiel Test conditions Test
15 17 20 19 14 16 21 22 23
Metal dissolved (%)
1st neutral stage
2rid acid stage
Temperature (°C)
Duration (min)
Temperature (°C)
Duration (min)
105 105 105 i 15 i 15 l !5 125 125 125
120 120 120 60 60 60 33 35 30
105 115 125 105 115 125 105 I 15 125
90 60 30 90 60 30 90 60 30
Zn
Cu
Pb
Ag
97.7 97.0 95.5 98.3 95.3 96.0 98.8 96.0 95.2
95.5 97.0 97.5 96.2 96.6 97.5 97.0 97.5 97.5
96.2 97.0 96.8 96.4 96.8 96.9 96.4 97.1 96.7
97.0 97.5 97.5 96.9 96.9 97.5 97.1 97.4 97.3
constants, and remains in solution in the form of ammines of these metals. The treatment of the pregnant solution to recover the dissolved metals is greatly facilitated as a consequence of this ammonia production. The treatment may take a particularly appropriate form by using solvent extraction involving acid cationic extractants. This type of process is known to generate an equivalent amount of acid to that of the metal extracted. The neutralization of this acid by the ammonia displaces the corresponding equilibrium reaction: Me(NH3).~ + +2C1- +2RH-,R2Me+2NH~- +2C!-
(6)
favouring metal removal. As indicated earlier, not all the ammonia required for extraction according to this reaction can be obtained from leaching. According to the global equations (2) and ( 3 ), which act jointly during leaching, the necessary amount of ammonia can be obtained only if the contribution of the latter reaction is nil. In practice this does not happen as the reaction given in eqn. (3) makes an important contribution. As a result, there is a high degree of contamination of the solution during leaching, due to the formation of sulphates which must be eliminated to avoid their accumulation in the leaching solution. The best means of eliminating the sulphates is to precipitate them as calcium sulphate. The use of lime produces ammonia: CaO+ 2NH~ + SO42- + H20-~CaSO4- 2 H 2 0 + 2NH3
(7)
which also provides the means of compensating for the ammonia deficit in
158
J.L. LIMPO ET AL.
the process. The appropriate dosage of lime and calcium chloride both eliminates the sulphates and supplies the required amount of ammonia. There is no particular complication in this precipitation step apart from the tendency of the gypsum to crystallize. In general, CaSO4-2H20 has a strong tendency to crystallize in a acicular form, leading to voluminous precipitates due to the interlinking of the needles. These precipitates adsorb large volumes of solution and may lead to large losses in the solid-liquid separation. The presence of small quantities of citric acid eliminates this tendency, and leads to compact precipitates with excellent decantability and filterability properties. The results obtained for the values of the equilibrium concentrations of calcium and sulphate at different temperatures (45-105 °C range) show that the solubility of calcium sulphate in ammonium chloride solutions is not greatly affected by the temperature, however, it does increase slightly with this variable. Finally, neutralization, with ammonia of the acid produced during extraction with cationic extractants, regenerates the original ammonium chloride solution, which is recycled to the leaching stage, thus closing the circuit. THE CENIM-LNETI PROCESS
Figure 7 is a block diagram of the CENIM-LNETI process for the treatment of sulphidic bulk concentrates.Leaching is undertaken in two countercurrent stages as described above. The most appropriate operating conditions for the Iberian concentrates are 105 °C and 150 kPa oxygen partial pressure. When the neutral leaching is terminated the operation takes place in the absence of oxygen, so that part of the remaining sulphide reduces the majority of the leached Cu ~+ to Cu + to prevent cupric diamine from precipitation during cooling. Following the solid-liquid separation, Ag and Hg are recovered by cementing with copper. Lead is separated as a chloride and crystallizes during the cooling of the solution to 50°C by vacuum evaporation. The lead chloride is redissolved and the lead is cemented from the resultant solution with granulated zinc or zinc powder. The concentrated zinc chloride solution resulting from the cementation stage is used to wash the zinc-loaded organic phase obtained during the extraction of this metal. The elimination of the sulphates generated during leaching is carried out by precipitating them with lime in the presence of citric acid. This operation is effected prior to zinc extraction and with a stoichiometric deficiency of lime. In this way the level of sulphates circulating in the solution is maintained, and this does not disturb any of the previous stages but is nevertheless beneficial in the two following extraction stages, as described below. The zinc is extracted with DEHPA at 50 ° C. In this operation, Ca and Cu (II) are co-extracted in quantities which may be large. Co-extraction is minimized
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by ensuring that the organic phase is loaded to values near to Zn saturation. At this stage it is also important that the majority of the copper be present in the unextractable cuprous form, and that there be a minimum level of sulphate in solution to maintain the Ca concentration at a low level. In any case, the loaded organic phase is washed in the zinc chloride solution (derived from lead cementation) to eliminate the co-extracted Cu and Ca. The organic phase is then washed with water to eliminate chlorides and, finally, goes through stripping with the Zn return electrolyte. Once the zinc has been extracted, the Cu + is oxidized with oxygen or air to Cu 2÷, which is then extracted with LIX 65 N. The resultant acid solution, after copper extraction, is recycled to the acid leaching stage. The copper extraction is favoured by the presence of sulphates because the equilibrium: SO~- + n ÷ ~ HSO~-
(8)
has a buffer effect which favours extraction. This process was tested on a laboratory scale with satisfactory results. The leaching, lead chloride and sulphate elimination processes were demonstrated on a batch scale, whereas the solvent extraction was performed continuously. CONCLUSIONS
The present article presents a concise description of the CENIM-LNETI process for the treatment of complex polymetallic sulphides and also summarizes the chemical principles on which it is based. The process has several characteristics which distinguish it from other processes. Its advantages make it particularly appropriate for the treatment of this type of ore. The following sets out the most significant characteristics and advantages of the process: ( l ) Solubilization of all important metal values (Pb, Cu, Zn and Ag) with a level of efficiency above 95%. (2) Production of a s 91ution which is totally free of iron, as a consequence of the high pH value at which leaching takes place. (3) Impurity elements, such as As, Sb, Bi and Sn, present in these concentrates and considered to be harmful in solution, are left almost entirely in the leach residue, and are detected in the solution only at trace levels. (4) The ammonia required for the solvent extraction treatment of the pregnant solution is provided by the process itself. ( 5 ) Regeneration of the leaching solution during solvent extraction. (6) Mild operating conditions. ( 7 ) Minimum consumption of reactants. (8) Favourable energy balance.
TREATMENT OF COMPLEX SULPHIDES IN AMMO ~,IUM CHLORIDE SOLUTIONS
161
ACKNOWLEDGMENTS
The researchers gratefully acknov,]edge the financial assistance provided by the Commission of the EC.
REFERENCES 1 Limpo, J.L. and Luis, A., 6th Asambl. Ge~. del CENIM. (Madrid) (1985), Pap. 173. 2 Demanhe, J.M. and Georgeaux, A., In: M i~~.Jones (Editor), Complex Metallurgy 78. Inst. Min. Metall., London (1978), pp. 113-12'~. 3 Guy, S., Broadbent, C.P., Lawson, G.J. an ~lJackson, J.D.J., Cupric chloride leaching of a complex copper/zinc/lead ore, Hydromet~ lturgy, 10 ( 1983 ): 243-255. 4 Rath, P.C., Paramguru, R.K. and Jena, P. IiL, In: Proc. Australas. Inst. Min. Metall, 278 ( 1981 ): 33-38. 5 Andersen, E, Boe, G.H., Danielssen, T. an,!, Finne, P.M., In: M.J. Jones (Editor), Complex Sulphide Ores. Inst. Min. Metali., Lond ~,n (1980), pp. 186-192. 6 Craigen, W.J.S. and CANMET/MSL Staff, CANMET, Rep. MSL 89-67(OP), OItawa ( 1989 ). 7 Greig, J.A., In: G.A. Davies (Editor), Separat ion Processes in Hydrometallurgy. Ellis Horwood, Chichester (1987), pp. 34-48. 8 Smyres, G.A., In: Institute of Mining and Meta;;~urgy (Editor), Extraction Metallurgy. Inst. Min. Metall., London (1989), pp. 839-860. 9 Limpo, J.L., Luis, A. and Gomez, C., Reactio ~s during the oxygen leaching of metallic suiphides in the CENIM-LNETI process Hydro ,netallurgy, 28 ( 1992 ): 00-00. l 0 Limpo, J.L., Figueiredo, J.M., Amer, S. and Lui,,, A., Spanish Pat. 8.902.487 (1989).
149
Elsevier Science Publishers B.V., A m s t e r d a m
The C E N I M - L N E T I process: a new process tbr the hydrometallurgical treatment of complex sulphides in ammonium chloride so, utions J.L. Limpo ~, J.M. Figueiredo ~, S. Amer a and A. Luis ~ ~Centro Nacional de Investigaciones Metalurgicas (Ct_'NIM), C.S.L C., A venida de Gregorio del Amo 8, 28040 Madrid, Spain bLaboratorio Nacionai de Engenharia e Tecnoh~:~:iaIndustrial (LNETI), Lisbon, Portugal (Received January 24, 199 !; revised version accepted July 22, 1991 )
ABSTRACT Limpo, J.L., Figueiredo, J.M., Amer, S. and Luis, A., 1992. The CENIM-LNETI process: a new process for the hydrometallurgical treatment of complex sulphides in ammonium chloride solutions. lirdrometallurgy, 28:149-161. A new process for the hydrometallurgical treatment of complex sulphides is presented. This process is based on an oxidizing leach of the sulphides in an ammonium chloride medium, which produces ammonia which forms metal ammine complexes with ith.~ solubilized metals (Cu and Zn ). The leaching occurs at a practically constant and neutral pH, arn~ produces a solution which is totally free of iron. Impurity elements, such as As, Sb, Bi and Sn, aTre detected in the solution only at trace levels. The presence of ammonia in the fertile leaching solutions facilitates its treatment by solvent extraction with acid cationic extractants, a stage in which tl~e leaching solution is regenerated. The paper presents a basic, general description of the process, ill us !rated with the results obtained using one of the bulk concentrates analyzed.
INTRODUCTION
Spain and Portugal have large deposits of complex sulphides, where the sulphides of non-ferrous metals are thialy dispersed through a pyrite matrix with a high level of association. These structural characteristics make it difficult to take full advantage of differen'tial flotation, as it is practically impossible to achieve high recoveries and hitChgrades of the individual concentrates at the same time. One solution to this problem is the recovery of the individual metals through treatment of the bulk concentrate. The main difficulty in this approach lies in the complex prc,blem of the treatment of these polysulphide bulk concentrates. Although many processes have been reported in the past, the treatment may still be consi~lered to remain unsatisfactorily resolved. In recent years, CENIM in Spain I~ad LNETI in Portugal have been work0 3 0 4 - 3 8 6 X / 9 2 / $ 0 5 . 0 0 © 1992 Elsevier Scit :ice Publishers B.V. All rights reserved.
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J.L. LIMPO ET AL.
ing on this problem in a joint project sponsored by the EC. The result of this bilateral collaboration has been the development of a new treatment procedure, the CENIM-LNETI process, which proposes an answer to the problem that seems to have clear advantages over existing processes. The basis of the process is an oxidizing leach in a chloride medium. Although there is an extensive body of literature on chloride leaching, there is practically nothing published on the use of concentrated solutions of ammonium chloride as the leaching medium [ 1 ]. The use of ammonium chloride confers special advantages on the process, making it particularly appropriate for the treatment of complex sulphide concentrates. The present article provides a general description of the process and discusses some of the most important results of the treatment of different bulk concentrates. HYDROMETALLURGICAL TREATMENT IN AMMONIUM CHLORIDE SOLUTIONS
The high solubilizing power of the chloride medium has attracted a great deal of attention in the study of the hydrometallurgical treatment of sulphide ores, particularly for the treatment of complex sulphides. As a result, several processes have been reported in recent years which basically differ in the use ofdifferent oxidating agents: Cu 2+ [2,3 ]; Fe 3+ [4-6]; O2 [7,8 ]; C12 [8 ]; etc. However, all use a leaching medium with a high concentration of chlorides, which is normally achieved through the addition of sodium chloride or, less often, by the addition of calcium chloride or a sodium chloride-.calciurn chloride mixture. In all these processes, the saline additive p,'oviJes a sufficiently high concentration of chloride ion to guarantee the solubl!ity of lead and silver. l h e addition of calcium chloride permits the control of sulphates in the solution. The CENIM-LNETI process uses a concentrated solution of ammonium chloride as the leaching medium and oxygen as the oxidizing agent. As well as providing an adequate concentration of chloride ion, the addition of ammonium chloride has other functions which are equally important and distinguish this process from the others. These characteristics are derived from the weak acid nature of the ammonium ion according to the reaction: NH~ ~ H + +NH3
(1)
The use of ammonium chloride thus has two advantages. Firstly, like the other chlorides, it provides an adequate concentration of chloride ion and, secondly, the ammonium ion is able to generate the protons required for the leaching of the sulphides, as well as producing ammonia. The resultant ammonia forms highly stable complexes with the majority of common metals (Cu 2+, Cu +, Zn 2+, Ag+, Hg 2+, etc. ), and this increases the solubilizing power of this leaching medium.
TREATMENT OF COMPLEX SULPHIDES IN AMMONIUM CHLORIDE SOLUI IONS
15 l
In the oxygen leaching of non-ferrous sulphides at moderate temperatures ( < !50°C) in an acid medium, the sulphur is essentially oxidized only to its elemental state. On the other hand, when the leaching occurs in a basic medium, the oxidation continues up to the formation of sulphates. When leaching takes place in ammonium chloride solutions, the leaching medium has a pH value which is very close to neutral. Under these conditions both possibilities play an important role. The reactions and mechanisms by which there is an attack on the different minerals are complex, and are analyzed in detail elsewhere [ 9 ]. Dispensing with the intermediate steps, leaching in an ammonium chloride medium develops according to a competition between the following two global reactions: MeS+2NH~- + ½02--,S+Me 2+ +HEO+2NH3
(2)
MeS + 202 ~ SO42- + Me 2+
(3)
Thus, in addition to solubilizing the non-ferrous metals, the leaching reactions also produce ammonia and sulphates which enter the solution. One of the most significant characteristics of leaching in an ammonium chloride solution is the practically constant and almost neutral pH, generally in the 6-7 range. In spite oi" the continuous generation of ammonia in the solution, the pH values does not vary significantly because copper and zinc form highly stable ammonia complexes. As a result, the ammonia formed during leaching mostly becomes combined in the form of metal ammines, and only a small quantity of uncombined ammonia in equilibrium remains as free ammonia with a high concentration of ammonium salts. This constitutes a buffer system which keeps the pH practically constant. The most important consequence of leaching in a neutral medium is the high purity of the solution obtained. Following leaching, the pregnant solution with the metallic values to be recovered has extremely low amounts of impurities and, in particular, is totally free of iron. Due to the high pH value of the solution, all the iron contained in the attacked mineral species (sphalerite, chalcopyrite and pyrite) remains in the residue as hydrated ferric oxide. Other minor elements, such as As, Sb and Bi, also stay almost totally in the leach residue, and are detectable only in trace amounts in the solution. Finally, another important characteristic of this leaching process is the need for a small initial concentration of dissolved copper. Without this metal, the leaching rate falls considerably, and practically stops at temperatures below 105 ° C. This behaviour is interpreted as indicating that the attack of the sulphides is via Cu 2+ ions, while oxygen re-oxidizes the Cu + ion which is formed. CONCENTRATES
The development ofthe CENIM-LNETI process was carried out using three
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J.L. LIMPO ET AL.
TABLE 1 Composition of the concentrates Element
Zn Cu Pb Fe S As Sb Bi Ag
Analysis (%) Sotiel
Aznalcollar
Ajustrel
33.5 3.4 7.8 17.1 34.6 0.28 0.32 0.01 0.018
29.6 3.8 9.9 16.5 33.8 0.28 0.40 0.12 0.023
19.5 8.3 6.1 21.3 36.3 0.30 0.13 0.12 0.012
different bulk concentrates: the Spanish concentrates of Sotiel and Aznalcollar and the the Ajustrel concentrate from Portugal. Table 1 sets out the composition of the major elements in the three concentrates. X-ray diffraction analyses indicated that the concentrates have similar mineral compositions, primarily: sphalerite, chalcopyrite, galena and pyrite. Table 1 shows that the most significant difference is the greater copper content, and thus lower lead and zinc contents, in the Portuguese concentrate. EXPERIMENTAL RESULTS
Leaching in ammonium chloride solutions The leaching of complex sulphide concentrates with oxygen in concentrated solutions of ammonium chloride produces the easy solublization of the important metal values, provided that there is copper in the leaching solution. The leaching rate is high in the presence of copper, even under operating conditions of moderate oxygen partial pressures and temperatures. As an example, the results from this type of leaching are set out in Fig. 1, where the percentage of leached metal is shown as a function of time for Zn, Cu, Pb and Ag. The raw material used in this test was the Sotiel bulk concentrate, using a pulp density of 100 g concentrate/kg water. The rest of the experimental conditions were the following: 105°C, 100 kPa oxygen partial pressure and an initial solution composition of 321 g ammonium chloride/ kg water and 1.5 g copper/kg water. As seen in Fig. 1, the solubilization rate for Pb and Ag is very fast and is considerably more rapid than that of Zn and Cu. A possible cause of the higher rate is the smaller granulometry of the galena compared to that of the sphalerite and chalcopyrite in these concentrates, although this fact alone is not
TREATMENT OF COMPLEX SULPHIDES IN AMMONIUM CHLORIDE SOLUTIONS
153
100
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60
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u <
40
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~
20
3'o
9'o TIME (MIN)
Fig. !. Leached metals as a function of time for the Sotiel concentrate.
0 . 5
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0.3
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30
60
90
120
TIME ( M I N )
Fig. 2. Ammonia and sulphate produced versus time for the experiment outlined in Fig. 1.
sufficient to explain such a large difference. The maximum solubilization of Pb is reached in the first few minutes, after which there is a gradual fall in the percentage of solubilized metal as a function of time. This phenomenon has been repeated systematically in all the tests, and is a consequence of ce~ain hydrolysis reactions. Figure 2 shows the evolution with time of the ammonia and sulphate concentrations during the same test. The figure indicates that there is a continuous increasing trend in the sulphates, while the ammonia tends to reach a maximum value asymptotically. In order to clarify this effect, a series of tests was done with longer leaching times. It was found that a maximum value of ammonia in solution was reached and, thereafter, there was a fall in the amount of ammonia that intensified with increasing leaching time. This fall occurred as a result of the oxidation of the pyrite according to the reaction:
154
J.L. L I M P O ET AL.
FeS2 + 1 5 / 4 0 2
+
5/2H, O--,2H2 SO4
4- 1 / 2 F e 2 0 3 " H 2 0
(4)
The oxidation of pyrite occurs at a substantially lower rate than that of the non-ferrous sulphides. The reduction of the ammonia content thus begins only when a certain degree of exhaustion of these sulphides is reached. This occurs when the rate of ammonia production according to eqn. (2) is lower than that of acid production according to eqn. (4). Figures 3 and 4 represent the variation with time of the percentage of Zn and Cu leached at different temperatures in the 85-125 °C range. The remaining operating conditions were maintained constant at the following levels: oxygen partial pressure 100 kPa; pulp density 100 g concentrate/kg water; 1001. . . . . . . . . . . . . . . . .
N 60
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20
ZX n L
30
L
A
............
60 90 TIME (MtU)
95~C 8%"C A
...........
120
Fig. 3. Effect ot" temperature on zinc leaching kinetics. L,each conditions: !' o,= !00 kPa, [ NH,,CI ],=6.0 mol/kg !-i~O, [Cu ],= !.5 g/kg H~O and 100 g Sotiel bulk concentrate/kg H,O.
l O 0 ...............................................................................................................
8O u 60
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................ t ............................................................ L ..................................... A ......................
30
60
90
120
TIME (MIN)
Fig. 4. Effect of temperature on copper leaching kinetics. Leach conditions: Po,= 100 kPa, [ NH4CI ]0=6.0 mol/kg H20, [Cu ]o= !.5 g/kg H20 and 100 g Sotiel bulk concentrate/kg H20.
TREATMENTOF COMPLEXSULPHIDESIN AMMONIUMCHLORIDESOLUTIONS
155
ammonium chloride concentration 321 g/kg water; initial concentration of copper 1.5 g/kg water; and Sotiel concentrate. The leaching rate clearly in= creases with temperature, which is the most influential of all the variables. The results obtained for the other two concentrates show a similar behaviour except that higher leaching rates are obtained with the Ajustrel concentrate. This is possibly related to its higher chalcopyrite (i.e., copper) content. The percentage of solubilized Zn and Cu as a function of time is shown in Figs. 5 and 6, respectively, in a series of leaching tests at oxygen partial pressures ranging between 100 and 500 kPa. The temperature was maintained constant at 105 °C in all the tests. The influence of 02 partial pressure is obviously small. There is a slight increase in leaching rate when the pressure 100-80 r" U
,._, 60 I u
< 40
uJ _J
•
17
,2-,,-,
20 / /
............. _x. . . . . . . . . . . . .
30
_-i
'~
..
" :~"_~ II
"
I. O0
,,
t
60 90 TIME (MIN)
120
Fig. 5. Effect of oxygen pressure on zinc leaching kinetics. Leach conditions: T= 105°C, [ NH.ICI ]o=6.0 mol/kg HaO, [Cu ]o= 1.5 g/kg H20 and 100 g Sotiel bulk concentrate/kg 1-120. 100 .............................................................................................
8O
~ 6o
o
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l
,
;-o-o ~ 30o
•
500
30..............60". . . .910. . TIME (MIN)
"
,.-
Pr~^
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,,
,,
120L
Fig. 6. Effect of oxygen pressure on copper leaching kinetics. Leach conditions: T = I05°C, [NH4CI ]o = 6.0 m o l / k g H20, [Cu ]o = 1.5 g l k g 1-120) and I00 g Sotiel bulk concentrate/kg 1420.
156
J.L. LIMPO ET AL.
increases from 100 to 200 kPa, but beyond 200 kPa, oxygen partial pressure has no effect. The initial concentration of copper in solution has a negligible effect on the leaching rate provided a minimum of 1 g Cu/kg water is maintained. During leaching, a minimum degree of agitation is required to ensure the transfer of oxygen to the solution to oxidize the cuprous ion in accordance with its formation rate. An increase in stirring above this level has no influence on the leaching rate. However, a reduction in stirring below the critical level may have serious consequences because, when the oxygenation is incomplete, the copper is precipitated according to the following reaction: 2Cu + + S-,Cu 2+ +CuS
(5)
which stops the leaching reaction when all the copper is precipitated.
Acid and two stage leaching According to the results exposed above, the amount of ammonia produced during leaching reaches a maximum value, and then decreases with a prolongation of leaching time. The maximum value is reached for an average solubilization of the metal values ( Z n + C u + Pb) between 80% and 85% Beyond this point, when solubilization yield increases, the amount of ammonia begins to decrease; initially slowly, but beyond 90% yield faster. In the previous section lead solubilization was also seen to fall as the leaching time increased, because of certain hydrolysis reactions. These results indicate the advisability of a two-stage countercurrent leaching process involving an initial stage of neutral leaching in which the bulk concentrate is treated, and a second stage of acid leaching in which the residue from the first stage is exhausted. Table 2 sets out the results obtained using the Sotiel bulk concentrate in a series of two-stage leaching tests. The experiments were programmed in the basis of the results of the individual analyses of the neutral and acid leach. In all tests a oxygen partial pressure of 100 kPa was used. The temperature and time of each stage is indicated. In these tests, neutral leaching involved the treatment of the concentrate with the solution obtained from the previous acid leach, while the residue obtained in the first stage was treated to exhaustion in the acid leaching stage. In all cases, the acidity used was the stoichiometric equivalent of tile non-ferrous metals contained in the residues to be leached. The solubilization yields shown in Table 2 are calculated on the basis of the analytical values of the leach residues.
Treatment of the leaching solution One of the most interesting characteristics of the ammonium chloride leaching medium is the in situ production of ammonia. The ammonia combines with the dissolved metals according to the corresponding complexation
TREATMENT OF COMPLEX SULPHIDES IN AMMONIUM CHLORIDE SOLUTIONS
157
TABLE 2 Results of two-stage leaching tests Bulk concentrate: Sotiel Test conditions Test
15 17 20 19 14 16 21 22 23
Metal dissolved (%)
1st neutral stage
2rid acid stage
Temperature (°C)
Duration (min)
Temperature (°C)
Duration (min)
105 105 105 i 15 i 15 l !5 125 125 125
120 120 120 60 60 60 33 35 30
105 115 125 105 115 125 105 I 15 125
90 60 30 90 60 30 90 60 30
Zn
Cu
Pb
Ag
97.7 97.0 95.5 98.3 95.3 96.0 98.8 96.0 95.2
95.5 97.0 97.5 96.2 96.6 97.5 97.0 97.5 97.5
96.2 97.0 96.8 96.4 96.8 96.9 96.4 97.1 96.7
97.0 97.5 97.5 96.9 96.9 97.5 97.1 97.4 97.3
constants, and remains in solution in the form of ammines of these metals. The treatment of the pregnant solution to recover the dissolved metals is greatly facilitated as a consequence of this ammonia production. The treatment may take a particularly appropriate form by using solvent extraction involving acid cationic extractants. This type of process is known to generate an equivalent amount of acid to that of the metal extracted. The neutralization of this acid by the ammonia displaces the corresponding equilibrium reaction: Me(NH3).~ + +2C1- +2RH-,R2Me+2NH~- +2C!-
(6)
favouring metal removal. As indicated earlier, not all the ammonia required for extraction according to this reaction can be obtained from leaching. According to the global equations (2) and ( 3 ), which act jointly during leaching, the necessary amount of ammonia can be obtained only if the contribution of the latter reaction is nil. In practice this does not happen as the reaction given in eqn. (3) makes an important contribution. As a result, there is a high degree of contamination of the solution during leaching, due to the formation of sulphates which must be eliminated to avoid their accumulation in the leaching solution. The best means of eliminating the sulphates is to precipitate them as calcium sulphate. The use of lime produces ammonia: CaO+ 2NH~ + SO42- + H20-~CaSO4- 2 H 2 0 + 2NH3
(7)
which also provides the means of compensating for the ammonia deficit in
158
J.L. LIMPO ET AL.
the process. The appropriate dosage of lime and calcium chloride both eliminates the sulphates and supplies the required amount of ammonia. There is no particular complication in this precipitation step apart from the tendency of the gypsum to crystallize. In general, CaSO4-2H20 has a strong tendency to crystallize in a acicular form, leading to voluminous precipitates due to the interlinking of the needles. These precipitates adsorb large volumes of solution and may lead to large losses in the solid-liquid separation. The presence of small quantities of citric acid eliminates this tendency, and leads to compact precipitates with excellent decantability and filterability properties. The results obtained for the values of the equilibrium concentrations of calcium and sulphate at different temperatures (45-105 °C range) show that the solubility of calcium sulphate in ammonium chloride solutions is not greatly affected by the temperature, however, it does increase slightly with this variable. Finally, neutralization, with ammonia of the acid produced during extraction with cationic extractants, regenerates the original ammonium chloride solution, which is recycled to the leaching stage, thus closing the circuit. THE CENIM-LNETI PROCESS
Figure 7 is a block diagram of the CENIM-LNETI process for the treatment of sulphidic bulk concentrates.Leaching is undertaken in two countercurrent stages as described above. The most appropriate operating conditions for the Iberian concentrates are 105 °C and 150 kPa oxygen partial pressure. When the neutral leaching is terminated the operation takes place in the absence of oxygen, so that part of the remaining sulphide reduces the majority of the leached Cu ~+ to Cu + to prevent cupric diamine from precipitation during cooling. Following the solid-liquid separation, Ag and Hg are recovered by cementing with copper. Lead is separated as a chloride and crystallizes during the cooling of the solution to 50°C by vacuum evaporation. The lead chloride is redissolved and the lead is cemented from the resultant solution with granulated zinc or zinc powder. The concentrated zinc chloride solution resulting from the cementation stage is used to wash the zinc-loaded organic phase obtained during the extraction of this metal. The elimination of the sulphates generated during leaching is carried out by precipitating them with lime in the presence of citric acid. This operation is effected prior to zinc extraction and with a stoichiometric deficiency of lime. In this way the level of sulphates circulating in the solution is maintained, and this does not disturb any of the previous stages but is nevertheless beneficial in the two following extraction stages, as described below. The zinc is extracted with DEHPA at 50 ° C. In this operation, Ca and Cu (II) are co-extracted in quantities which may be large. Co-extraction is minimized
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by ensuring that the organic phase is loaded to values near to Zn saturation. At this stage it is also important that the majority of the copper be present in the unextractable cuprous form, and that there be a minimum level of sulphate in solution to maintain the Ca concentration at a low level. In any case, the loaded organic phase is washed in the zinc chloride solution (derived from lead cementation) to eliminate the co-extracted Cu and Ca. The organic phase is then washed with water to eliminate chlorides and, finally, goes through stripping with the Zn return electrolyte. Once the zinc has been extracted, the Cu + is oxidized with oxygen or air to Cu 2÷, which is then extracted with LIX 65 N. The resultant acid solution, after copper extraction, is recycled to the acid leaching stage. The copper extraction is favoured by the presence of sulphates because the equilibrium: SO~- + n ÷ ~ HSO~-
(8)
has a buffer effect which favours extraction. This process was tested on a laboratory scale with satisfactory results. The leaching, lead chloride and sulphate elimination processes were demonstrated on a batch scale, whereas the solvent extraction was performed continuously. CONCLUSIONS
The present article presents a concise description of the CENIM-LNETI process for the treatment of complex polymetallic sulphides and also summarizes the chemical principles on which it is based. The process has several characteristics which distinguish it from other processes. Its advantages make it particularly appropriate for the treatment of this type of ore. The following sets out the most significant characteristics and advantages of the process: ( l ) Solubilization of all important metal values (Pb, Cu, Zn and Ag) with a level of efficiency above 95%. (2) Production of a s 91ution which is totally free of iron, as a consequence of the high pH value at which leaching takes place. (3) Impurity elements, such as As, Sb, Bi and Sn, present in these concentrates and considered to be harmful in solution, are left almost entirely in the leach residue, and are detected in the solution only at trace levels. (4) The ammonia required for the solvent extraction treatment of the pregnant solution is provided by the process itself. ( 5 ) Regeneration of the leaching solution during solvent extraction. (6) Mild operating conditions. ( 7 ) Minimum consumption of reactants. (8) Favourable energy balance.
TREATMENT OF COMPLEX SULPHIDES IN AMMO ~,IUM CHLORIDE SOLUTIONS
161
ACKNOWLEDGMENTS
The researchers gratefully acknov,]edge the financial assistance provided by the Commission of the EC.
REFERENCES 1 Limpo, J.L. and Luis, A., 6th Asambl. Ge~. del CENIM. (Madrid) (1985), Pap. 173. 2 Demanhe, J.M. and Georgeaux, A., In: M i~~.Jones (Editor), Complex Metallurgy 78. Inst. Min. Metall., London (1978), pp. 113-12'~. 3 Guy, S., Broadbent, C.P., Lawson, G.J. an ~lJackson, J.D.J., Cupric chloride leaching of a complex copper/zinc/lead ore, Hydromet~ lturgy, 10 ( 1983 ): 243-255. 4 Rath, P.C., Paramguru, R.K. and Jena, P. IiL, In: Proc. Australas. Inst. Min. Metall, 278 ( 1981 ): 33-38. 5 Andersen, E, Boe, G.H., Danielssen, T. an,!, Finne, P.M., In: M.J. Jones (Editor), Complex Sulphide Ores. Inst. Min. Metali., Lond ~,n (1980), pp. 186-192. 6 Craigen, W.J.S. and CANMET/MSL Staff, CANMET, Rep. MSL 89-67(OP), OItawa ( 1989 ). 7 Greig, J.A., In: G.A. Davies (Editor), Separat ion Processes in Hydrometallurgy. Ellis Horwood, Chichester (1987), pp. 34-48. 8 Smyres, G.A., In: Institute of Mining and Meta;;~urgy (Editor), Extraction Metallurgy. Inst. Min. Metall., London (1989), pp. 839-860. 9 Limpo, J.L., Luis, A. and Gomez, C., Reactio ~s during the oxygen leaching of metallic suiphides in the CENIM-LNETI process Hydro ,netallurgy, 28 ( 1992 ): 00-00. l 0 Limpo, J.L., Figueiredo, J.M., Amer, S. and Lui,,, A., Spanish Pat. 8.902.487 (1989).